More mutation histograms

[savitakartik]

It would be useful to show histograms of (a) number of inheritors per mutation, (b) number of descendants per mutation. Perhaps we might also want to plot (c) the ratio of inheritors to descendants per mutation? This may be useful, for example in the case of SARS-CoV-2 genomes, to flag specific mutations for investigation, such as those with very few inheritors but lots of descendants.